Astronomers spot cold dust in hot gas of ancient galaxies

U-M
astronomers have detected the cool infrared signature of dust grains
and silicates within superheated gas in the center of ancient
elliptical galaxies 60 million light-years from Earth.

The discovery of these particles is significant, according to
astronomer Joel Bregman, because it could represent the first direct
observation of how mass lost by aging stars evolves in a hot, exotic
environment.

Bregman and his colleagues found unexpected evidence for the
existence of dust and silicates in infrared emissions from nine
elliptical galaxies. Results from their research were presented Jan.
6 at the American Astronomical Society meeting. Spectral data for the
study were collected by the Infrared Camera (ISOCAM) on the European
Space Agencys Infrared Space Observatory.

Since the Earths atmosphere blocks most infrared radiation,
space observatories like ISO allow astronomers to observe objects too
cold or faint to be seen in visible light. Another advantage of
infrared light is that it passes undisturbed through gas and dust
clouds allowing astronomers to see inside dense,
star-forming areas in the center of galaxies.

Elliptical galaxies contain stars that are five- to
15-billion years old, which is about as old as stars can be in the
universe, said Bregman, professor of astronomy. Space
between the stars in these galaxies is filled with superheated X-ray
emitting gas that is about 10 million degrees Kone of the
hottest environments in the universe.

Temperatures are comparable to those inside a supernova
remnant, but on a galaxy-wide scale, added graduate student
Alex Athey. Even the hottest parts of our Milky Way Galaxy
rarely exceed 1 million degrees K.

Because gas in the center of the observed galaxies is so hot,
Bregman and his colleagues were startled when an excess in their
infrared spectral data indicated the presence of material
significantly colder than the surrounding gas.

We expected to see the normal starlight spectrum, along with
polyaromatic hydrocarbons or PAHs, Bregman said. When we
didnt find what we expected, we asked ESA for additional
observations from ISOCAM in the six- to 15-micron wavelength band.
ESA went the extra mile and gave us additional observation time with
higher resolution detail.

Although Bregman and his colleagues are still interpreting the
data, they believe the source of the unusual infrared emissions is
dust grains and silicates thrown off by stars interacting with
superheated gas inside elliptical galaxies.

Stars lose mass during their evolution, primarily when they
are giant stars, Bregman said. As this gas flows away
from the stars, some of the material forms into grains of
dustsomewhat smaller than particles in cigarette smoke. This
material is exposed to the very hot gas of the galaxy which slowly
destroys the grains, and to UV light of the galaxy, which heats the
dust grains causing them to emit in the mid-IR region. If this
interpretation is accurate, additional analysis of infrared emissions
from elliptical galaxies could provide important clues to how
galaxies evolve.

Collaborators in the study included Jesse Bregman of NASA and
Pasquale Temi of the NASA Ames Research Center. Funding for the study
was provided by NASA.